It is not practical for a combine harvester to return to a grain storage location every time its grain tank is full. Instead, an unloading tractor is driven alongside the harvester and the grain is unloaded from its grain tank while both vehicles are moving. As there may be several harvesters operating near one another and several unloading tractors servicing them by ferrying the grain from the harvesters to the storage location, coordinating the movement of all these vehicles is a complex logistical exercise that can be better performed if the vehicles are able to communicate wirelessly with one another.
The communication system between the vehicles needs to take the following requirements into account, namely:
1. The wireless communication performance must not be significantly affected by multiple vehicles in the same transmission area (e.g. bandwidth reduction due to multiple communications),
2. Detection (discovery) of the vehicles that may provide/request support. For example, a combine harvester may request to be unloaded, and a tractor may provide the unloading service.
3. The operators must not be constantly distracted and irritated by messages enquiring with which vehicle they would like to work. Instead the system should be as autonomous as possible in detecting which pair of vehicles may cooperate in a joint operation.
4. The communication bandwidth must not be significantly affected by multiple connection procedures between vehicles moving in and out of the transmission/reception area.
U.S. Pat. No. 7,555,283, which is believed to represent the closest prior art to the present invention, is directed to a communication system for mobile and stationary devices, which act as receivers and sources of information, respectively, comprising at least one transmitter/receiver unit provided for the mobile and stationary devices and having a defined transmission/reception range, and also formed so that at least partial overlap of the transmission/reception ranges of the mobile and/or stationary devices results in accidental communication between the mobile and/or stationary devices, and this accidental communication changes to a targeted communication, wherein communication comprises a direct exchange of information between the sources and receivers and the communication system is formed so that during the accidental communication, an application context between the communicating mobile and/or stationary devices is credit (SIC) and, if an application context of this type is not present, the accidental communication is terminated.
The latter patent classifies individual communications between two devices as “accidental” and “targeted”. An accidental communication is one between two devices that may or may not be intending to cooperate with one another. If the exchanged data meets certain criteria defined by an “application” then an accidental communication changes into a targeted communication. The unloading of harvesters is given as one example of an “application” and in this example, once a targeted communication is commenced, the unloading tractor is controlled by the harvester.
Of the four requirements set forth above, the communication system of U.S. Pat. No. 7,555,283 meets those numbered 2 and 3, but it suffers from the disadvantage of overloading the available bandwidth. The resultant risk of interference makes it unsafe to have several targeted communications taking place simultaneously on the same communication channel.